This invention is directed to the field of liquid-type photoresists and the process of electrophoresis for depositing such films on conductive surfaces.
Photoresists are photosensitive films capable of transferring an image onto a conductive surface such as, for example, the metal surface of a printed circuit board or lithographic plate. Liquid-type photoresists typically contain a combination of a film forming resin or polymer and a photosensitive compound or photoinitiator dissolved or suspended in a solvent such as an organic liquid.
Liquid-type photoresists can be negative-acting or positive-acting systems. In the case of a negative-acting photoresist or negative resist, after the film is deposited on a surface and the solvent is removed as by heating, the film is selectively exposed, typically through a photomask, to a source of energy, such as ultraviolet light. The photomask has areas that are opaque and other areas that are transparent to the exposing radiation. The pattern on the photomask formed by the opaque and transparent areas defines the desired image, such as, for example, a circuit, which is to be transferred to the substrate surface. The exposed portions of a negative resist film become less soluble in a developing solution, as the result of a photochemical reaction between the photoinitiator and the polymer or resin upon exposure, than the unexposed portions. This difference in solubility allows for the selective removal of the unexposed film and the transfer of the image to the surface. In a positive resist, the exposed portions of the film become more soluble in the developer than the unexposed portions, as a result of the photochemical reaction, allowing for the selective removal of the exposed areas. After either type of resist film is developed, the portions of the surface that are not protected by the resist may be etched, such as by the action of an oxidizing solution typically containing an inorganic acid. The remaining resist film may then be stripped from the surface leaving only the desired etched image on the substrate. Alternatively, the substrate surface containing the imaged resist can be plated with a metal or combination of metals, such as, for example, tin and lead. The resist may then be selectively stripped and the exposed metal on the substrate may be etched to form the desired pattern or a circuit on the substrate surface. The historical background, types and operation of conventional photoresists is described in Photoresist Materials and Processes, W. S. DeForest, McGraw-Hill, 1975.
Although liquid-type resists have been used for many years in lithographic and electronics applications, and despite numerous improvements to the resist systems and processing steps involved with their use, these conventional liquid resists still suffer from one or more disadvantages. For example, special surface preparations may be required to obtain good adhesion of a liquid resist film onto a surface. This increases the processing time and cost involved. The resists themselves may also require other special processing steps, such as hardening or baking steps, which also increase processing time. The cost of the resist components utilized in conventional systems combined with the loss of materials in depositing a film and the difficulty in reproducibly synthesizing stable systems, has also been a problem. The most widely recognized drawback of liquid resists, however, has been the difficulty to deposit films of uniform and adequate thickness on surfaces without the formation of voids or pinholes. Furthermore, there remains a need for a resist having resistance to etching and electroplating baths that are functional over a wide range of exposing radiations at minimal exposure doses and times. The use of organic solvents to formulate and develop liquid resists also presents a potential health, flammability and environmental concern, as well as adding to the potential for swelling the images formed and reducing the image resolution achievable.
Despite the abundance of conventional liquid type photosensitive coatings, dry film type resists are also important in the preparation of printed circuit boards. Dry film resists are multilayer films in which the photoresist is precast into a solid film and sandwiched between a polyethylene film and a polyester cover sheet. Dry film resists are described in U.S. Pat. Nos. 3,469,982, 4,378,264 and 4,343,885. The dry film resists are typically laminated onto the surface of a printed circuit board, as by application with hot rollers, after the bottom polyethylene film layer is peeled from the adjacent photoresist layer and top cover sheet. The transparent cover sheet, which provides mechanical support for the photoresist film layer, is usually kept in place until after the photoresist is exposed through a photomask and the cover sheet. After exposure, the cover sheet is peeled away from the resist film and the resist is developed and processed in a conventional manner.
Despite the advantages of these dry film resists over liquid type resists, such as the ability to apply a uniformly thick coating without pinholes, dry film resists also suffer from a number of drawbacks. Typically the conductive copper surface of a printed circuit board contains a protective coating, known as a chromate conversion coating, that protects the copper from oxidation. Before a dry film resist can be applied to the metal surface, the conversion coating must be removed as by abrasive scrubbing. This scrubbing roughens the surface and facilitates adhesion of the dry film resist to the substrate surface. However, the need for an abrasive scrubber can lead to defective circuitry on subsequent etching operations. Dry film resists may also have difficulty in adhering to metal surfaces unless the metal has been specially prepared. In addition, dry film resists cannot be used effectively with surfaces having irregular topographies. For instance, a small scratch in the substrate surface may not be filled by a dry film resist. The dry film will merely bridge the scratch and can allow etchant to seep into the scratch during etching resulting in unacceptable circuits. Furthermore, in addition to the high cost of dry film resists, their use also results in substantial waste of materials in the form of unusable strips of excess film because the dry film resists are typically trimmed to conform with the substrate surface to be coated.
A second aspect of the present invention relates to the process of electrophoretic deposition. Electrophoresis refers to the motion of charged particles or molecules through a liquid medium under the influence of an applied electrical field. Electrophoretic deposition or electrodeposition is conducted in an electrolytic cell where the surface of the conductive material, to be coated by the migrating charged particles, serves as one electrode. Polymers bearing a positive charge in the liquid medium, or polymers which become associated with a substance such as a surfactant having a positive charge, are known as cationic polymers. The electrodeposition of cationic polymers on the surface of a negatively charged electrode (cathode) is referred to as cataphoresis, while the electrophoretic deposition of negatively charged polymers (anionic polymers) onto the surface of a positively charged electrode (anode) is known as anaphoresis.
Coating metal objects with organic materials by electrophoresis is well known and is widely used to paint metal surfaces such as automobiles. Electrophoresis has been used to prepare electrical components, such as resistors and capacitors, that are integral to printed circuit boards (U.S. Pat. No. 3,303,078). U.S. Pat. No. 3,403,088 is directed to anaphoresis of water dispersed acrylic interpolymers to prepare thermally curable, non-photoactive, insulating coatings for electrical devices. U.S. Pat. No. 3,446,723 is also directed to thermally cured, non-photoactive, coatings preferably applied by cataphoresis. Cataphoresis is taught in this patent to be superior to anaphoresis because nascent oxygen does not react with the polymers. In addition, metal ions are eliminated from passing into solution resulting in a decrease in the water repellancy and discoloration of the coatings. Other patents disclosing the use of electrophoresis to deposit non-photoactive, thermally curable coatings include: U.S. Pat. Nos. 3,925,181; 3,975,251; 3,200,057; 4,238,385; and 4,338,235. Polymer Compositions for Cationic Electrodepositable Coatings, P. E. Kondomenos and J. D. Nordstrom, Journal of Coatings Technology, Volume 54, No. 686, March 1982, pages 33-41 also describes recent advances in this coating area.
Electrophoretic deposition of photosensitive coatings is also generally known. U.S. Pat. No. 3,738,835 describes the use of anaphoresis to deposit a photosensitive composition from an emulsion prepared from a solution containing a polychloroprene polymer, a photosensitizer, such as 4,4'-bis-(dimethyl-amino)benzophenone, a stabilizer, such as hydroquinone that prevents the unsaturated polymer from decomposing in solution, and a hardener such as a partially cured resin or other polymer to provide etch resistance, in an 80% butyl acetate 20% methyl ethyl ketone or 80% cyclohexanone 20% methyl ethyl ketone solvent. The emulsion is formed by the addition of an aqueous solution containing a wetting agent, such as a fluorocarbon anionic surfactant, N-methyl-2-pyrolidone and triethyl amine to the first solution. Upon exposure of the photosensitive composition to radiation, the unsaturated polymer crosslinks and the unexposed film is developed with an organic solvent. U.S. Pat. Nos. 3,954,587; 4,029,561; 4,035,273; 4,035,274; 4,039,414; 4,066,523; 4,070,258 and 4,166,017 also disclose coatings that are applied by electrophoresis and are cured by exposure to ultraviolet (UV) light and/or heat. These coatings are intended to provide a permanent protective coating or to improve the appearance of the article to be coated. These patents do not disclose or suggest that such coatings are useful as photoresists and in fact they are not suitable for such use. U.S. Pat. No. 3,844,919 relates to the electrodeposition of a permanent photoconductive material prepared from a photoconductive zinc oxide, an electrically insulating resin, such as an acrylic resin, and an organic solvent or water.
U.S. Pat. No. 4,414,311 describes the preparation of lithographic printing plates by cataphoretically applying a thin, submicron photosenstive film, from an aqueous solution of a polymeric material having pendant diazonium salts, onto an electrochemically hydrophilized metal surface. The thin film is exposed to UV radiation, developed and inked to form a lithographic printing plate.
Japanese patent application No. 77-11601 describes electrophoretically depositing a photosensitive polymer composition onto a silicate coated metal surface. The photosensitive polymers are soluble or dispersible in an aqueous medium and capable of being anaphoretically deposited on a conductive surface by virtue of containing a neutralized acid groups. The polymers are photosensitive by virtue of containing unsaturated groups that crosslink upon exposure to light. The preparation of these polymers requires at least two steps; either the addition of an acid to a polymer having unsaturated groups or the addition of unsaturated groups to a polymer having acid groups. These resins are illustrated as being formed from an acrylic polymer having pendant hydroxy groups and an isocyanate-containing methacrylate compound. The preparation of these photosensitive polymers is difficult because of the number of separate reactions needed. In addition, the extent to which such polymers can be effectively crosslinked is limited by the number of available unsaturated groups on the polymer or the number of unsaturated groups that can be attached thereto. The number of unsaturated groups that can be attached to the polymer may also effect other desirable properties of the resist such as its aqueous solubility and capability to form high quality films. In addition, photosensitive compositions containing a neutralized acid group are not capable of being used in a cataphoretic deposition process and while the patent mentions cataphoresis, it fails to suggest any composition that can be used to cataphoretically form an aqueous acid developable and strippable resist that is resistant to aqueous base solutions and inorganic acids.
Japanese patent disclosure No. 55(1980)-148491 entitled Method for Preparing Printed Wiring Boards discloses using electrophoresis for coating photosensitive substances onto the copper surface of a copper-laminated plate in preparing printed circuit boards, however, this reference does not describe any materials useful for the process or any of the process conditions used therein.
It is an object of the present invention to provide a photosensitive polymeric composition, capable of being deposited directly onto a conductive surface from an aqueous solution or emulsion by electrophoresis, that solves the problems of conventional liquid-type resists and dry film resists.
It is also an object of the invention to provide a photosensitive polymer composition that can be electrophoretically deposited as a uniform film of desired thickness on a conductive surface of regular or irregular surface topography without the need for elaborate and time consuming surface preparation steps and without wasting the photosensitive composition.
It is a further object of the invention to provide a photosensitive polymer composition that can be cataphoretically deposited on a conductive surface as an aqueous base and inorganic acid resistant film than can be developed with organic acids to form highly resolved images.